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The design is heavily inspired by pedestal.interceptor & sieparri, both excellent libraries. They are also quite opinionated, in ways that didn't fit our usage and the core of the concept being around hundred lines we decided to roll our own, with its own specificities.

It mimics pedestal interceptor behaviour, but adds async lib agnostic support with implementations for manifold, core.async, CompletableFuture. It differs from pedestal by removing suppressed, terminators, not wrapping errors at every throw level and not catching throwable. Pedestal implementation is also much larger and deeply tied to core.async.

If you're familiar with sieparri our library differs by following pedestal style error handling instead (modulo wrapping/rethrowing) and by not having a :request/:response backed in, we just have context in -> context out, or whatever you call as termination, from there we think you can emulate other usage patterns more easily. Sieparri also differs in a few way at the API level to accomodate the its usage goals but these are minor differences.

How it works:

It follows the interceptor pattern.

execute takes a context (map) and chains it to the "interceptors" that can modify it and ultimately returns a value or the context itself.

An interceptor is just a map, with :enter key and potentially :leave :error and :name (+ whatever you want to add).


:enter and :leave are function of the context and return a new context. :error takes 2 arguments, the context and the error that triggered it and return potentially a new context.

When you execute it will pass the context to the :enter handlers first, they return a new updated context; then the :leave handlers in reverse (if present).

Something like that:

enter A -> enter B -> enter C -> leave C -> leave B -> leave A


Upon encountering an error it will trigger the next :error handler available, so either one on the interceptor that triggered it, if there is one, or the next one on the stack. If you just return the context from the :error handler it will resume with the execution of the stack with that context; if you rethrow the error, or assoc it/one under ::error (or via (error ctx err)) in the context and return it, it will trigger the next error handler available on the stack.

The rule of thumb is to use error handlers to manage errors that should trigger a termination of the processing and have control on the flow of that termination (instantaneous return or propagating). You should still use normal error handling in your :enter handlers if you want to let the normal flow of execution continue (ex if you need to go to the next :enter handler).

This works exactly to what's defined in the original interceptor pattern.

Manifold, core.async, CompletableFuture support

You can mix steps that return deferreds/CompletableFutures/Channels with normal steps like it will work transparently.

note: if you use the normal execute the first async interceptor in the chain will dict the return value type (if it starts with a deferred you get a deferred back). You can control the return values via exoscale.interceptor.manifold/execute, exoscale.interceptor.core-async/execute and exoscale.interceptor.auspex/execute and they still allow you to mix internal steps with whatever lib you like/use.


You can apply lenses/guards on handlers via in out lens when discard functions.

They are just middlewares to the handlers so you can apply them separately to the handlers and have full control over their execution order.

Interceptors definition

You can define interceptors as maps as described earlier but we also by default allow to specify them in other formats/types.

  • Keyword -> {:enter k}
  • Function -> {:enter f}
  • Symbol -> resolved to anything that should implement the Interceptor protocol
  • Var -> deref'ed to something that should implement the Interceptor protocol


(def interceptor-A {:name :A
                    :enter (fn [ctx] (update ctx :a inc))
                    :leave (fn [ctx] (assoc ctx :foo :bar))
                    :error (fn [ctx err] ctx)})

(def interceptor-B {:name :B
                    :enter (fn [ctx] (update ctx :b inc))
                    :error (fn [ctx err] ctx)})

(def interceptor-C {:name :C
                    :enter (fn [ctx] (d/success-deferred (update ctx :c inc)))})

(def interceptor-D {:name :D
                    :enter (fn [ctx] (update ctx :d inc))})

(execute {:a 0 :b 0 :c 0 :d 0}

;; because we have an async step it will return a deferred
=> << {:a 1, :b 1, :c 1, :d 1, :foo :bar} >>

;; no async step, direct result
(execute {:a 0 :b 0 :d 0}

=> {:a 1, :b 1, :d 1, :foo :bar}

;; lens

(execute {:a 0}
         [{:name :foo :enter (lens inc [:a])}])
=> {:a 1}

;; same using in/out

(execute {:request 0}
         [{:name :foo
           :enter (-> inc
                      (in [:request])
                      (out [:response]))}])
=> {:request 0 :response 1}

;; guard/when

(execute {:a 0}
         [{:name :foo
           :enter (-> (fn [ctx] (update ctx :a inc))
                      (when #(contains? % :a)))}])
=> {:a 1}

;; discard output, just return ctx

(execute {:a 0}
         [{:name :foo
           :enter (-> (fn [ctx] (prn :yolo))
=> {:a 0}

Implementing custom interceptor types

Interceptors creation is behind a protocol so if you keep writing the same "lenses" all the time you might be better of writing something tailored to your needs.

For instance for an hypothetical interceptor type that would pull from/to :request/:response in the context so you can just define them as normal ring handlers.

(defrecord RingInterceptor [enter]
  (interceptor [{:as i :keys [enter]}]
    (cond-> i
      (assoc :enter (fn [ctx]
                      (assoc ctx :response (enter (:request ctx))))))))

;; then
(execute {:request {:params ...}} [(RingInterceptor. (fn [request] {:body "yolo"}))])

You can imagine holding state/dependencies at that level too if that's something you desire (that's doable with context too).

Api docs

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Logo is cc licensed The author is "mckinney75402"


Small Interceptor lib for clojure







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